Which vendors have you used for providing incentive gifts? In our case we want to offer a gift card for completing a survey. The survey is anonymous and we want to be able to have a link at the end which the respondent can click on to claim their gift card.

I haven't done this specifically, but you can purchase digital gift cards from many vendors and may be provided with a list of the codes to send as confirmation of completing the survey. If you're looking to dynamically generate/include the link in the confirmation email, there may be someone else who has worked with a vendor that offers this.

Rybbon is used by 25+ Marketo customers for sending digital gifts like e-gift cards and Visa pre-paid cards via Marketo. We have unique integration for adding gifts directly to Marketo emails and logging lead activity when gifts are claimed.